Eleven laboratories participated in the First International Workshop of Granulocyte Serology. Participants were asked to identify any granulocyte-reactive antibodies present in 12 human sera distributed as unknown samples. Granulocyte immunofluorescence and granulocyte micro-agglutination were the most commonly used techniques. The incidence of false-positive and false-negative reactions was 15 and 4%, respectively. Most laboratories were able to distinguish between granulocyte-specific and anti-HLA antibodies using platelet absorption. The identification of the specificity of granulocyte-specific antibodies caused the most problems; these were correctly identified on only 57% of occasions.